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| Section Two: What is Fiber Optics? pg.3 |
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Multimode
fiber generally uses inexpensive LED light sources,
although lasers can also be employed. These LED’s
are unfocused, and will emit light rays at many
angles.
The core of a multimode fiber is large enough, in
relative terms, that when the LED blinks, multiple
rays enter the core and travel the length of the
fiber. Some of the light rays will travel close
to the center of the core, and some will bounce
from the core/cladding boundary on one side to the
other, as illustrated.
“Multi mode” means multiple paths. There are many
paths that a light ray can take in a multimode core
to reach the other end.
Multimode fiber functions like a shotgun. Multiple
pellets are sent down the barrel, and come out the
other end. |
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As
can be seen in the illustration, light rays traveling
near the center will arrive at the receiving end
first, followed by the light rays that are bouncing
from cladding to cladding. Because the shortest
distance between two points is a straight line,
the center or “close coupled mode” light rays will
arrive before the outer or “loose coupled” rays.
Because of this difference in arrival times of the
light rays, the delivery of a light ray to the receiver
can be spread out over time, causing slowing of
communication. If the blink of the LED is 3 nanoseconds
at launch, the same blink may lengthen to 6 or 7
nanoseconds at the receiving end of the multimode
fiber. As the receiver can only count one blink
at a time, the communication slows. This is called
“pulse spreading”, and is the primary factor that
limits multimode fiber to approximately 8 kilometers
or 5 miles in usable link length. |
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| LESSON
REVIEW |
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Key
Points:
1.
There are many paths that a light ray can take when travelling
in a multimode fiber core.
2.
"Multimode" means multiple paths.
3.
"Pulse Spreading" limits the distance that multimode
fiber can be successfully used.
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Pop
Quiz:
True or False
When many light rays are sent into a multimode fiber core
at the same time, they will all reach the receiver at the
same instant.
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Key
Terms:
1.
Pulse Spreading
2.
Close coupled rays
3.
Loose coupled rays
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